The skin is the largest organ in the human body and consists essentially of two primary layers—the epidermis and the dermis. The epidermis is the outermost layer and, among other things, controls water loss from cells and tissue. The dermis is the layer below the epidermis and contains blood vessels, lymph vessels, hair follicles and sweat glands. Below the dermis is the hypodermis. Although the hypodermis is considered to be part of the integumentary system, it is not generally considered to be a layer of the skin. The hypodermis is used mainly for fat storage.
The outermost epidermis is made up of stratified squamous epithelium with an underlying basement membrane. It contains no blood vessels, and is nourished by diffusion from the dermis. The main type of cells that make up the epidermis are keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum, granulosum, spinosum, basale. Cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the corneum and become sloughed off. This process is called keratinization and takes place within about 30 days.
The dermis consists largely of the protein collagen, which forms a network of cross-linked fibers providing a framework for blood vessels and cell growth. Because it is the primary component of the dermis, collagen acts as the support structure for the skin. The health and stability of collagen is a critical factor in determining the contour, wrinkles and lines in the skin.
Collagen, a naturally occurring fibrous protein found in both humans and animals, provides structural support for bones, tendons, ligaments, and blood vessels, in addition to its role in the skin. Collagen is the most abundant protein in the body.
There are several major types of collagen, which give rise to the variety of structural and functional properties that collagen exhibits throughout the body. With age or injury, the collagen in a person begins to weaken and lose its elasticity. In the skin, this process eventually results in the appearance of wrinkles.
The basic structural unit of a collagen fiber is tropocollagen. It consists of a triple helix of three intertwined peptide chains of approximately 1000 amino acid residues. The basic polypeptide unit of the peptide chain is a repeating sequence of 3 amino acids, where every third residue is a glycine, and the other two alternate between proline and hydroproline. It is important to the stabilizing feature of the collagen fiber that the glycine residue is every third residue because its small side chain allows for tight coiling of the three helices, providing a strong stabilizing structure.
In young skin, the collagen remains intact and elastic, however, as the skin ages, the support structure weakens, the skin loses elasticity and the collagen support wears down from the cumulative stress of, for example, facial expressions. This causes lines and wrinkles to appear in the skin.
Collagen replacement therapy can be used to treat conditions associated with the breakdown or loss of collagen. For example, skin wrinkles can be treated by injecting highly purified collagen into the dermis. Injection of collagen has also been used to soften scar tissue and create fuller lips.
Current collagen replacement therapies include collagen injections in which purified animal collagen is used to replace lost tissue. Zyderm® and Zyplast® are bovine collagen implants that are injected into the dermis. There they become incorporated into the human collagen framework and replenish the skin's natural collagen thereby restoring the support structure and the contour of the skin. This injection therapy enhances and improves the natural appearance of skin and smoothes facial lines and scars.
Procedures involving injecting collagen are not without risk. For example, bovine collagen injections can cause allergic reactions such as redness, swelling, firmness, itching and, in rare instances, abscess formation. Worse, some physicians have reported the occurrence of connective tissue diseases such as rheumatoid arthritis, systemic lupus erythematosus, dermatomyositis (DM), and polymyositis (PM) subsequent to collagen injections in patients with no previous history of these disorders.
Also, the injection process itself poses certain challenges. For example, the practitioner injecting collagen (and/or hyaluronic acid) must control the depth, orientation and position of the needle at a particular injection site, while providing an inward force on the plunger that is sufficient to force a controlled flow rate of high viscosity collagen out of the needle and into the exact location in the dermis that will provide the desired cosmetic effect. The locating of the needle tip at the proper depth within the dermis is also difficult for the practitioner. To engage the tip of the needle at the proper injection depth, the practitioner may move the needle inwardly and outwardly with respect to the surface of the skin (epidermis). However, there is no visual reference point, other than the end of the syringe body, from which the practitioner can easily determine the extent that the needle extends into the dermis. Thus, the needle tip may be placed too deep, or too shallow, for the intended application. It should be appreciated that the person (practitioner) injecting the collagen must have good, steady control of the fingers, hand and arm and also have excellent eye-hand coordination to be an effective provider of cosmetic collagen injections. These qualities are not always present in individuals, and this has limited the availability of collagen therapy to patients.
In addition to injection, collagen may be delivered to the skin by topical application. Unfortunately, such topical collagen therapy has proven less than effective as conventional forms of collagen do not appear to penetrate into the dermis. As noted above, the skin consists of multiple layers and is extremely complex in terms of its function as well as its chemical make-up. Transdermal (through the skin) application of medicines and other substances poses a wide range of formulation hurdles. The ability to deliver desired substances to a layer within the skin is equally, if not even more, difficult.
Various means for delivery of substances to or into the skin have been proposed.
U.S. Pat. No. 5,354,564 discloses personal care products comprising an aqueous dispersion of particles of silicone wherein said particles have a surface modifier adsorbed on the surface thereof in an amount sufficient to achieve a particle size of less than about 400 nanometers (nm).
U.S. Pat. No. 5,660,839 discloses incorporating deformable hollow particles into cosmetic and/or dermatological compositions containing fatty substances, for markedly reduce or eliminate the sticky and/or greasy feel attributed to these fatty substances.
U.S. Pat. No. 5,667,800 discloses an aqueous suspension of solid lipoid nanoparticles, comprising at least one lipid and preferably also at least one emulsifier, for topical application to the body.
U.S. Pat. No. 5,780,060 discloses microcapsules with a wall of crosslinked plant polyphenols and compositions containing them. The microcapsules are obtained by the interfacial crosslinking of plant polyphenols, particularly flavonoids.
U.S. Pat. Nos. 5,851,517 and 5,945,095 disclose compositions including a dispersion of polymer particles in a non-aqueous medium. A dispersion of surface-stabilized polymer particles can be used in a non-aqueous medium, in a cosmetic, hygiene or pharmaceutical composition. The dispersions may, in particular, be in the form of nano-particles of polymers in stable dispersion in a non-aqueous medium.
U.S. Pat. Nos. 5,759,526 and 5,919,487 disclose nanoparticles coated with a lamellar phase based on silicone surfactant and compositions containing them. The nanoparticles, and in particular nanocapsules, provided with a lamellar coating obtained from a silicone surfactant, can be used in a composition, in particular a topical composition, for treatment of the skin, mucosae, nails, scalp and/or hair.
U.S. Pat. No. 5,188,837 discloses a microsuspension system and method for its preparation. The microsuspension contains lipospheres which are solid, water-insoluble microparticles that have a layer of a phospholipid embedded on their surface. The core of the liposphere is a solid substance to be delivered or a substance to be delivered that is dispersed in an inert solid vehicle such as a wax.
U.S. Pat. No. 4,919,841 discloses a process for preparing encapsulated active particles by the steps of: dispersing active materials in molten wax; emulsifying the active/wax dispersion in an aqueous surfactant solution for no longer than 4 minutes; quenching the capsules by cooling; and retrieving solidified capsules. Examples of active materials are fragrances.
Each of these methods has disadvantages, particularly with respect to the delivery of collagen and/or hyaluronic acid.
Liposomes are vesicular lipid membrane structures that enclose, for example, a volume of water. The existence of liposomes has been known for many years. In the early 1900's, researchers, studying isolated lecithin (phosphatidylcholine), cephalin (phosphatidylethanolamine/phosphatidylserine), phrenosin (galactosyl ceramide) and kerasin (glucosyl ceramide), found that all of these molecules would swell in water to form hydrated multilamellar layers, consisting of lipid bilayers separated by water. Also, mixtures of ionic and nonionic lipids dispersed in water were found to form stable “emulsions” in which the lipid molecules take up positions side by side to form a homogeneous mixed phase. These emulsions were the equivalents of what are now called multilamellar liposomes.
Physical and chemical studies have shown that amphiphiles form certain preferred arrays in the presence of water. Formation of these arrays, which include micelles, monolayers and bimolecular layers, is driven by the need for the polar head groups, which may be ionogenic or not, to associate with water and the need of the apolar, hydrophobic tail to be excluded from water. Exactly which type of structure is assumed depends upon the nature of the amphiphile, its concentration, the presence of other amphiphiles, temperature, and presence of salt and other solutes in the aqueous phase.
Until recently, liposome technology has been concerned mostly with vesicles composed of phospholipids, predominantly phosphatidylcholine, and these continue to be the focus of most publications and patents. However, although phospholipids are suitable for certain pharmaceutical applications, phospholipid liposome technology has been beset by serious problems, for example, phospholipids turn over rapidly in vivo and are unstable in storage. Also, they are labile and expensive to purify or synthesize, and the manufacture of phospholipid liposomes is difficult and costly to scale up.
Although liposomes are well known in the art, there are no previous reports of their use to efficiently deliver collagen and/or hyaluronic acid in a skin care formulation.